A dielectric resonator filter having a high Q value has been widely utilized as a high power filter for transmission for a base station in mobile communication at frequencies in an 800 MHz (Mega Hertz) to 1.5 GHz (Giga Hertz) band. A filter is necessary for each channel. An apparatus which houses sixteen filters for 16 channels as a group is usually used. Each filter is cooled by air or water for use because it has an insertion loss of about 40% when ten or more Watts are input.
A junction box type power filter apparatus which is used for the transmitting portion of the base station has a structure shown in FIG. 13 (see "THE BASIS OF MOBILE COMMUNICATIONS": The institute of electronics, information and communication engineers by Yoshihisa Okumura et. al. pages 255 to 256). In FIG. 13, 16-channel transmitters having varying frequencies within a frequency band are connected to resonator filters 542 through isolators 541. Each resonator filter 542 is formed by a dielectric resonator and is designed in such a manner that a signal within a band ranging from one to one hundred kilohertz (Khz) sent from the connected transmitter passes therethrough.
In consideration of space and installation costs, an antenna is usually shared in mobile communication. For this reason, a junction box is used. The junction box merges the outputs of a plurality of bandpass filters having different passband frequencies and close resonant frequencies to share an output transmission line. The junction box according to the prior art is connected to the output end of a microwave filter which uses a cavity resonator, a dielectric resonator or the like, and utilizes a coaxial line or the like.
In FIG. 13, a junction box 543 is used in order to connect 16-channel signals to a line while performing impedance matching. As shown in FIG. 13, branch lines formed by coaxial cables are hierarchically connected in the junction box 543. More specifically, the branch lines 544 having a length of .lambda./4 (i.e., approximately one quarter wavelength) are connected to the output sides of 16 resonator filters. Four branch lines 544 are collected into one and connected at four merging points. Branch lines 545 having a length of .lambda./2 are connected to the merging points. Two branch lines 545 are collected into one and connected at two merging points. Furthermore, branch lines 546 having a length of .lambda./2 are connected to the two merging points. The two branch lines 546 are connected to each other at a merging point to which the output line of a super power circulator. The super power circulator is connected to an antenna through a bandpass filter for preventing transmission of spurious signals outside the pass band.
The operating principle of the junction box is as follows. As seen from the merging point, the filters which resonate take impedance matching and other resonators are short-circuited. Accordingly, if outputs are merged at a distance of (2n+1).lambda./4 from the output end of the filter, the impedance of the branch line in the non-resonant state becomes infinite at the merging point. If the output passes through the branch line which is at a distance of (m+1).lambda./2 from the merging point, the impedance is not changed. Consequently, a mismatching loss is not caused even if the outputs of signals are merged again at the same position. As a result, transmission lines having a low loss can be shared. .lambda. is a wavelength at the average resonant frequency of the filter (electric length), and n and m are 0 or natural numbers.
The impedance of each resonator filter seen from the merging point of the coaxial cable having a length of .lambda./4 is equal to the characteristic impedance of an output line for the frequency of a passband, and is considerably increased for other frequencies. Consequently, the characteristics of other filters are hardly influenced. Consequently, the outputs of the resonators can be merged easily.
Referring to the filter apparatus according to the prior art, dielectric resonator filters for the number of channels are housed in a housing unit, and signals from the filters are merged to an output line by a junction box formed by coaxial cable branch lines. Consequently, the size of the apparatus is increased.
In a high power filter apparatus, a plurality of plane circuit type filters are formed by normal conductive metal electrodes which are not merged by the junction box and which have a coaxial line structure according to the prior art and which are operated at an ordinary temperature because of a low Q value and a high insertion loss. Even if such a filter apparatus can be put to practical use, each filter should be mounted on a shield case and connected to the junction box by a connector. Consequently, the size of the apparatus is increased and a connecting loss at each connector is caused.
While a plane circuit filter which forms a filter electrode with a superconductive thin film that has recently been developed is characterized by a high Q value, a low loss and a high output, it should be cooled to a low temperature to operate in the superconductive state. Conventionally, a superconductive filter unit housed in a shield case is fixed to a cryostat cold head to execute evaluation experiments on a laboratory level. For example, the shield case is connected to a semi-rigid cable by means of a general SMA type connector or the like and the other end side of the semi-rigid cable is connected to a connector provided on the wall of a heat insulating container so that the signal of the filter is input and output. However, an example in which a plurality of superconductive plane circuit filters are used to form the high power filter such as a mobile communication base station or the like has not been reported.
In a filter apparatus which houses a plurality of dielectric resonator filters, a power loss is great. For example, an insertion loss is 2 to 3 decibels (dB) at a frequency of about 1.5 GHz. Consequently, it is necessary to house the filter apparatus in a large-sized rack to be cooled by air or water. The dimension of each dielectric resonator is about 100 millimeter (mm) diameter (.phi.).times.120 mm height (H). The 16-channel filter portion has a width of 25 centimeter (cm), a depth of 25 cm and a height of 1 meter (m). If a space in which a cooling system for air or water cooling is included, the dimension of the whole filter apparatus is increased more. Furthermore, the power consumption for a predetermined transmitted power is so great that the filter apparatus is not economical.
When a resonant frequency is changed due to a change in temperature of the filter, there is a possibility that a mobile communication system will become hard to operate. Consequently, temperature stability is a necessary design constraint of the filter apparatus which generally increases the costs of the filter apparatus.
In addition, the channel frequency adjustment/tuning mechanisms should be provided at every filter which results in increased costs.
A semi-rigid cable, a coaxial cable for high power or the like is cut into pieces having a length of several cm are used for a branch line. Consequently, it is hard to manufacture a connecting portion. It is necessary for the skilled artisan in the art to manually perform processing, measurement and adjustment. For this reason, the junction box is disadvantageous to characteristic repeatability, manufacturing, costs and the like.
In the case where microwaves having a high power are incident on a superconductive filter according to the prior art and are merged by the junction box using a metal in the vicinity of a normal temperature, the temperature of a superconductive filter element becomes higher than that of the connector portion because the junction box generates heat due to a conductor loss so that normal conductive transfer (quench) is partially caused. Consequently, the normally conductive transferred portion is broken instantly.
Even if the junction box is formed by a small-sized cable material such as a semi-rigid cable to cool the whole filter unit, the size of the filter apparatus is limited by the minimum bend radius of the semi-rigid cable and cannot be reduced. In addition, a heavy current flows to the cable and the filter connecting portion so that the temperature of the superconductive thin film is increased due to Joule heat generated by a contact resistance loss and a conductor resistance loss. Thereby, a critical current value is decreased so that the maximum output power is limited.
As described above, if the junction box is formed by using the coaxial line or the like, the size of the apparatus is increased and a large amount of heat supplied from the coaxial line is removed. For this reason, the size of the whole apparatus including a necessary large-sized freezer is considerably increased and becomes more expensive.